1. Field of the Invention
The present invention relates to an optical pickup and more particularly to an optical pickup which is devised to allow the resonance frequency band of a base member on which a light emitting element and a collimator lens are installed to deviate from a vibration frequency band generated in association with the operation of a lens holder having an objective lens.
2. Description of the Related Art
FIG. 8 shows a plan view of an example of the schematic configuration of an optical disk apparatus using an optical pickup 1. In the drawing, the optical pickup 1 includes a base member 2 on which a lens holder 3 equipped with an objective lens 31 is installed and is controlled to access (or is controlled to travel to) an optical disk D along a radial direction thereof when bearings 21 and a rail bearing 22 which are provided on the base member 2 are caused to travel while being guided, respectively, by a guide shaft 52 and a guide rail 53 which are provided on a frame-shaped drive chassis 51. In addition, a rack 23 provided on the base member 2 meshes with a threaded shaft 54 mounted on the drive chassis 51, so that the rotational motion of the threaded shaft 54 is translated into the linear motion of the base member 2 by the rack 23. Furthermore, the drive chassis 54 is mounted on a frame 56 which is raised and lowered via a cam mechanism 55. The frame 56 is fixed to a stationary frame 57 such as a box body. Then, the optical disk D as a recording medium is mounted on a turntable 58 set on the frame 56 for rotation.
FIG. 9 is an explanatory drawing of an optical system of the optical pickup. As shown in the drawing, the optical system of the optical pickup includes the aforesaid objective lens 31 in addition to a light emitting element 32 made up of a light emitting diode or the like, a light receiving element 33, a half mirror 34 and a collimator lens 35, and the objective lens 31 is equipped on the lens holder 3, as has been described above, whereas the other components including the light emitting element 32 are mounted on the base member 2. Then, laser light emitted from the light emitting element 32 is guided to the collimator lens 35 by the half mirror 34 so as to become a flux of parallel light, which then transmits through the objective lens 31 so as to be emitted onto the optical disk D, and reflected light therefrom is received by the light receiving element 33 via the objective lens 31, the collimator lens 35 and the half mirror 34.
In this optical pickup, in order to correct a change in the positional relationship between the optical disk D and the objective lens 31 which would occur in association with the warp or eccentricity of the optical disk D, the lens holder 3 is controlled to be displaced in a focusing direction and a tracking direction relative to the optical disk D by the action of a driving unit 36. In general, the driving unit 36 is made up of a combination (not shown) of a supporting unit for supporting the lens holder 3 on the base member 2 in such a manner as to be displaced in the two directions, a coil and a magnet.
In the optical pickup, a proper vibration frequency used when the lens holder 3 is controlled to be displaced in the focusing direction and the tracking direction is adjusted so as to be accommodated within a certain band. Here, the vibration frequency of the lens holder 3 which has been properly adjusted is referred to as an adjusted frequency.
There is a case that the base member 2, on which the lens holder 3 which is controlled to be displaced in the focusing direction and the tracking direction is installed, resonates with the vibration of the lens holder 3 which occurs when the lens holder 3 is controlled to be displaced. Then, when the base member 2 resonates, the vibration of the base member 2 is then transmitted to the turntable 58 via the guide shaft 52 and the guide rail 53, the drive chassis 51 and the frame 56, which were described above by reference to FIG. 8, whereby since the optical disk D vibrates when the turntable 58 vibrates, the servo properties of the lens holder 3 are reduced, leading to the reduction in reliability in writing and reading by the optical pickup 1. Then, in the event that in the base member 2, the resonance frequency band overlaps the adjusted frequency of the lens holder 3 or resides in the vicinity thereof, even if the adjusted frequency has been set properly, the servo properties of the lens holder 3 are reduced, and no satisfactory value cannot be obtained for the writing and reading performance of the optical pickup 1, thereby resulting in the failure of adjustment.
The resonance of the base member 2 occurs prominently when the base member 2 is made of a thin synthetic resin molded body. FIG. 7B shows a gain curve and a phase curve in the focusing system when the base member 2 is made of a thin synthetic resin molded body, and in FIG. 7B, the resonance generated when the base member 2 resonated (vibrated) was transmitted to the turntable 58 to thereby cause the optical disk D to vibrate, and as a result, the gain curve or the phase curve was made to fluctuate irregularly in a range of 1 to 2 kHz (ranges where the irregular fluctuation occurred are indicated as G1, P1). Thus, when the gain curve or the phase curve of the base member 2 is caused to fluctuate irregularly in the range of 1 to 2 kHz, the vibration of the base member 2 is transmitted to the turntable 58 to thereby increase the vibration of the optical disk D, whereby the aforesaid adjustment is made difficult, possibly resulting in the reduction of focusing servo properties.
FIG. 10 is a schematic plan view of the base member 2 made of a synthetic resin molded body which is used in an optical pickup in the related art, and FIG. 11 is a view as seen in a direction indicated by an arrow XI in FIG. 10. On this base member 2, a light emitting element made up of a laser diode, not shown, is mounted and a radiator plate 7 made of a sheet metal is provided for radiating heat generated in the light emitting element. The radiator plate 7 includes a mounting piece 71 which is formed by bending part of the radiator plate 7 at right angles, a surface plate portion 72 and a projecting piece 73, and the mounting piece 71 is screwed to a mounting wall (not appearing in the drawing) of the base portion 2 at two locations with machine screws together with a radiator block 74 which is mounted on the light emitting element while the mounting piece 71 is being superimposed on the radiator block 74. In the conventional example, while the radiator plate 7 is screwed only to the mounting wall of the base member 2 with the machine screws, a collimator lens mounting hole (a light passage hole) 27, which is opened in a substantially central portion of a base plate portion of the base member 2, is covered with the surface plate portion.
Then, although the problem with the resonance of the base member 2 made of a synthetic resin molded body was attempted to be solved by devising the countermeasure in which the reinforcement rib is provided on the base plate portion 28 of the base member 2, since the mounting hole 27 is opened in the substantially central portion of the base plate portion, only with the reinforcement rib, a vibration mode such as bending and twisting is generated in the base member 2, and the resonance frequency band of the base member 2 which is generated when the lens holder 3 (refer to FIG. 8) is controlled to be displace has been unable to be made to deviate largely from the aforesaid adjusted frequency. Note that the gain curve or the phase curve which was explained above by reference to FIG. 7B is such as to be obtained as to the base member 2 constructed as has been described heretofore.
On the other hand, as to the optical pickup, there has been proposed a technique in which with a view to increasing the servo properties for access control by increasing the resonance frequency which deforms a fixing portion where a carriage and a holding member of a lens holder are fixed together, the rigidity of a location in the vicinity of the fixing portion is increased (refer to JP-A-9-102133). According to this technique, a cover obtained by pressing a sheet stainless steel is described as being fixed to upper surfaces of the carriage and a tracking coil. In addition, in an optical pickup device, with a view to suppressing the transmission of a reaction force of vibrations generated from a correction driving unit to a supporting body of a lens holder, there is proposed a means in which a supporting base and the supporting body are bonded together directly below a yoke functioning as the correction driving unit (refer to JP-A-2002-42356). Furthermore, it is described as a known example in which in an optical pickup device, optical system constituent components such as a semiconductor laser and a light receiving element are installed on a device frame made of a resin (refer to JP-A-2001-325740). Moreover, it is known that the optical system of an optical pickup is made up of a semiconductor laser, a collimator lens, a half mirror, a photo diode and the like (refer to JP-A-2002-352445)